Effects of Ethylene-Chlormequat-Potassium on Leaf Nitrogen Assimilation after Anthesis and Early Senescence under Different Planting Densities The Effects of Gibberellic Acid(GA3) on the Factors Scavenging Active Oxygens in the Flag Leaves of Hybrid Rice during the Later Growth Stages A COMPARATIVE STUDY ON DIFFERENT SENESCENT TYPES IN WHEAT VARIETY Physiological Characteristics and Gene Mapping of a Leaf Early-Senescence Mutant osled in Rice